DRY Mutual Friendships / Friends in Ruby on Rails

friendship.rb

# app/models/friendship.rb

class Friendship < ApplicationRecord
  belongs_to :user
  belongs_to :friend, class_name: 'User'
end

friendships_query.rb

# app/queries/friendships_query.rb

module FriendshipsQuery
  extend self

  def both_ways(user_id:)
    relation.unscope(where: :user_id)
      .where(user_id: user_id)
      .or(relation.where(friend_id: user_id))
  end

  private

  def relation
    @relation ||= Friendship.all
  end
end

user.rb

# app/models/user.rb

class User < ApplicationRecord
  has_many :friendships,
    ->(user) { FriendshipsQuery.both_ways(user_id: user.id) },
    inverse_of: :user,
    dependent: :destroy

  has_many :friends,
    ->(user) { UsersQuery.friends(user_id: user.id, scope: true) },
    through: :friendships
end

users_query.rb

# app/queries/users_query.rb

module UsersQuery
  extend self

  def friends(user_id:, scope: false)
    query = relation.joins(sql(scope: scope)).where.not(id: user_id)

    query.where(friendships: { user_id: user_id })
      .or(query.where(friendships: { friend_id: user_id }))
  end

  private

  def relation
    @relation ||= User.all
  end

  def sql(scope: false)
    if scope
      <<~SQL
        OR users.id = friendships.user_id
      SQL
    else
      <<~SQL
        INNER JOIN friendships
          ON users.id = friendships.friend_id
          OR users.id = friendships.user_id
      SQL
    end
  end
end